treacherous

From Old French trecheros, tricheros (“deceitful”), equivalent to treacher +‎ -ous. See treacher.

• IPA^(key): /ˈtɹɛt͡ʃəɹəs/, /ˈtɹɛt͡ʃɹəs/

• Audio (US):

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treacherous (comparative more treacherous, superlative most treacherous)

1. Exhibiting treachery.

   Antonym: loyal

   • 1855, Walt Whitman, Leaves of Grass:

     I see the wife misused by her husband, I see the treacherous seducer of young women, / I mark the ranklings of jealousy and unrequited love attempted to be hid […]

2. Deceitful; inclined to betray.

   Synonyms: Judaslike, perfidious, traitorous; see also Thesaurus:deceptive, Thesaurus:treacherous

3. Unreliable; dangerous.

   Synonyms: hazardous, perilous, unsound; see also Thesaurus:dangerous

   a treacherous mountain trail

   • 1959 March, “New Railways in Quebec”, in Railway Magazine, page 202:

     Work on the muskeg resulted in the formation in the first instance, and later in lengths of newly-laid track disappearing without trace into the treacherous bog.

   • 2002, Charles Morris, The San Francisco Calamity:

     […] they stood in the region of perpetual snow, amidst the glittering, treacherous glaciers and crevasses, with vast slippery-pathed precipices yawning round.

   • 2025 October 21, Rose George, “‘I knew in my head we were dying’: the last voyage of the Scandies Rose”, in The Guardian‎^[1]:

     It is standard practice for search and rescue authorities to ask other vessels in the area to assist. Usually, this is done. The weather on the night of 31 December was too atrocious, and when at 11pm the Coast Guard asked the crabber Ruff & Reddy to head to the scene, its skipper refused, as a skipper has a right to do if he believes conditions to be too treacherous.

• “treacherous”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.
• William Dwight Whitney, Benjamin E[li] Smith, editors (1911), “treacherous”, in The Century Dictionary […], New York, N.Y.: The Century Co., →OCLC.
• “treacherous”, in OneLook Dictionary Search.